The design of exoskeletons poses a complex challenge that goes beyond traditional robotic systems, as the interaction between humans and exoskeletons must be taken into account. In contrast to controlled robots, exoskeletons must be seamlessly integrated into the human musculoskeletal system, which requires appropriate control strategies and an understanding of human motion dynamics. In this talk, I will review the three main methods for developing exoskeletons: 1) Test and rebuild, 2) Human in the loop, and 3) simulation. I will mainly focus on simulation, which includes models of both the electromechanical system and the biomechanics of the human user.